Moineau style hydraulic motors and pumps are employed in subterranean drilling and artificial lift applications, such as for oil and/or gas exploration. Such motors make use of hydraulic power from drilling fluid to provide torque and rotary power, for example, to a drill bit assembly. While downhole drilling motors fall into the general category of Moineau-type motors, they are often subject to greater working loads, temperatures, and more severe chemical and abrasive environments than Moineau motors and pumps used for other applications. As such, the demands on drilling motor components (e.g., rotor and stator components) may far exceed the demands on the components of other Moineau-type motors and pumps. For example, drilling motors may be subject to a pressure drop (e.g., from top to bottom across the motor) of up to 1500 psi at temperatures of up to about 200° C. Furthermore, the stator may exceed 25 feet in length; thus achieving suitable processability (e.g., flowability) in order to form via injection mold the elastomer materials tends to be difficult at such lengths. Moreover, many rubber compounds are known to deteriorate or degrade in the presence of hydrocarbons.
The power section of a Moineau style motor may include a helical rotor disposed within the helical cavity of a corresponding stator. When viewed in circular cross section, a typical stator shows a plurality of lobes in the helical cavity. In various Moineau style power sections, the rotor lobes and the stator lobes are disposed in an interference fit, with the rotor including one fewer lobe than the stator. Thus, when fluid, such as a drilling fluid, is passed through the helical spaces between rotor and stator, the flow of fluid causes the rotor to rotate relative to the stator (which may be coupled, for example, to a drill string). The rotor may be coupled, for example, through a universal connection and an output shaft to a drill bit assembly. Rotation of the rotor therefore causes rotation of the drill bit in a borehole.
One drawback with stators having an all elastomer helical cavity component is that a tradeoff in elastomer properties has been required. One such tradeoff has been between the resilience (e.g., rigidity) of the elastomer and its processability (e.g., its flowability during injection molding). For example, as discussed in U.S. Pat. No. 6,905,319, processability is generally inversely related to the stiffness of the rubber, which is particularly true in injection-mold processes. A stiffer compound may demand much more processing power and time, thereby increasing manufacturing costs. As a result, conventional wisdom in the art suggests that rigid elastomers (e.g., those having a Shore A hardness of about 90) are not suitable for use in downhole stators due to inherently poor processability, and it is preferred to use elastomeric materials in conventional stators that have a hardness (Shore A) in the range from 65-75.
One significant drawback with conventional stators is that the elastomer helical cavity component deforms under torque loads (due in part to the low rigidity of the elastomer). This deformation creates a gap on the unloaded side of the stator lobe, thereby allowing drilling fluid to pass from one cavity to the next without producing any work (i.e., without causing rotation of the rotor). This is known in the art as “RPM drop-off.” When the torque reaches a critical level, substantially all of the drilling fluid bypasses the stator lobes and the rotor stalls. Thus, flexibility of the liner may lead to incomplete sealing between the rotor and stator such that available torque may be lost when the rotor compresses the stator lobe material, thereby reducing the power output of the positive displacement motor (PDM). Stiffer or harder rubbers may limit deformation, but may also restrict the sealing properties.
Additional problems may be encountered with stators when, for example, rotation of the rotor within the stator shears off portions of the stator lobes. This process, which may be referred to as “chunking,” deteriorates the seal formed between the rotor and stator and may cause failure of the PDM. Chunking may be increased by swelling of the liner or thermal fatigue. Swelling and thermal fatigue may be caused by elevated temperatures and exposure to certain drilling fluids and formation fluids, among other factors.